Boiler materials

[Jonathan HowesParticipant @jonathanhowes93303]

I am curious as to the apparent restrictions on boiler materials. I am new to this forum but (unfortunately…..) not new to engineering. The conventional materials are ordinary steels and copper with some justified suspicion of stainless steel but, I note, not an absolute exclusion.

 

I have built several high pressure air tanks for model aircraft compressed air engines in the past using thin phosphor bronze sheet (up to 160 lb/in^2 on 3″ diameter using .014″ sheet, design hoop stress safety factor >5 for example), often by careful riveting after a basic soft-soldered assembly, ie, the soft solder provides lap shear strength and allows a tight joint to be set prior to riveting but very low joint creep resistance and the rivets provide the actual long-term structural capability.

 

I am familiar with this method of construction and since it seems to offer the potential for a very near scale boiler would like to consider it for a locomotive. On the plus side it is incredibly easy to get a very high quality joint and corrosion resistance is outstanding even in horrible environments (often used for high strength marine fittings for example). On the down side it is clearly unfamiliar as a boiler material within the ME community other than for fittings.

 

The boiler code does not actually seem to preclude this (and seems to me to be an entirely reasonable document…. OK, I am an aeronautical engineer by background and worked on the development of aircraft structural requirements for a number of years, maybe I have a bias!).

 

What will, or should, stop me from doing this?

 

Jon.

Edited By Jonathan Howes on 13/04/2010 17:11:32

[Jonathan HowesParticipant @jonathanhowes93303]

[JasonBModerator @jasonb]

Rivited and caulked copper boilers are certainly possible but you have to take into account the loss of strength of the material and caulk when heated, not something your air tanks have to withstand.

 

You would also have to hydralic test to twice working pressure so again your airtank would need to stand upto 320psig

 

Typical thickness for a 3″ dia copper boiler barrel would be in the region of 0.063″ and thats for seamless tube so likely a little thicker with a lap joint.

 

You would also have to get any testing done by a professional boiler tester as clubs can only test copper and steel boilers. The boiler inspector would also want to see full calcs for the design.

 

Jason

Edited By JasonB on 13/04/2010 17:35:42

Edited By JasonB on 13/04/2010 17:48:24

Edited By JasonB on 13/04/2010 17:58:01

[mgjParticipant @mgj]

Yes, and have you seen the literally eggshell thin MARAGING steel rocket motor casings. They are so thin they literally wobble like a kiddies soap bubble. I forget how may bar they will stand, nor am I going back to revist the maths, but it is very very many. so yes, you can go very thin if you want to.

 

(for KWIL – think in numbers like Bar psig! – oops)

 

However you still have fittings and associated bushes to thread and fit, and the thing still has to have effective functionality in use/service. (Like racing suspensions. You make wishbones out of 22 gauge tube, and you replace more because of spanner rash than you do through failure) Many boilers are structural members, and so the limitation is external stresses, not pressure imposed hoop loads – and even then the boiler is often clad, so its not, in most practical senses actually visible.

 

So one has to ask what is one gaining?. If the thing is externally to scale dimension, a bit of extra internal copper will make no difference to function. So for me you are gaining delicacy and a very high servicing load for no improvement in functionality which doesn’t seem like a very good deal.

 

 

 

Edited By mgj on 13/04/2010 18:16:12

[pcb1962Participant @pcb1962]

by careful riveting after a basic soft-soldered assembly, ie, the soft solder provides lap shear strength and allows a tight joint to be set prior to riveting but very low joint creep resistance and the rivets provide the actual long-term structural capability.

 I would have thought that the use of soft solder for any purpose in the construction of a boiler was a very bad idea.

[JasonBModerator @jasonb]

The soft solder is only there as a caulk – to stop leaks, it is not structural, the rivits take care of that. But as mgj says you will need to add bushes etc which will really need silversoldering and any trace of soft solder in the area will stop the silversolder sticking.

 

Its a method that was used more in the past when a parafin blowlamp was all a lot of model engineers had and may have struggled to get a boiler upto temp for sil sold

 

Jason

 

Jason

[mgjParticipant @mgj]

You could use high temperature solder which is the same stuff that they use in fusible plugs.

 

As I understand it mixing silver solder and lead based solder is very dangerous because it will make a silver soldered joint very brittle.  – though that wouldn’t apply if you soft soldered after silver soldering.

 

There is no doubt that riveting is very strong. A proper aviation type 3 row joint with all drilled properly to correct size is IIRC stronger than a weld. However one isn’t going to use such a joint. One is trying to go “super” scale using scale thickness metal, and then promptly use an unoriginal material.

 

However this is not simply a pressure vessel – you do actually have to use it. Lay a fire, rake ash, operate handwheels, firedoors open and shut. Its going to look like a bit of bent wriggly tin in 5 minutes. And with respect (and I claim no great expertise) but you have actually seen a model boiler fire running – with the blower fully open, with the firebox really hot, and the roar of the draught – or after a bit of a run under load you are closing dampers to lower boiler pressure and cut the draught because you don’t need the heat. This is not to get distorted (the inner firebox) with such thin thicknesses? First time I got my TE going I was staggered at the intensity of the fire. Jason I’m sure knows exactly what I’m talking about.

 

Locos are no different : its not like a beam engine tottering along at 20psi.

 

Its a lovely idea, but I doubt it’s practical in a working model loco. The question is one of risk. You spend some time making the thing, and assembling it into a loco, and the inner firebox bends/distorts….. bit of a pisser really.

[Jonathan HowesParticipant @jonathanhowes93303]

Thanks guys, great responses. I do not have much experience of locos in the field hence the “beginners” category. I am a bit of a scale pedant in other areas so was looking for a construction that would allow the correct external appearance of lap/butt strap joints although I realise that this would result in a need for careful handling. Probably wildly unrealistic for a club knockabout type of operation. My interest is in really early engines so the performance would be hopeless for this class of operation in any case.

 

In summary it appears that yes, I could go this way but I need to do a bit of work on fitting attachment and don’t expect it to result in a useful model for general use. I will also need to get  specialist support to get a boiler certificate.

 

I am not therefore entirely discouraged since this is what I hoped would be the picture, it is a self-indulgent hobby after all!

 

Many thanks,

 

Jon. 

Edited By Jonathan Howes on 13/04/2010 21:08:39

[mgjParticipant @mgj]

Jonathan, I don’t think you should be discouraged at all. There’s no such thing as can’t, but there may well be a different way.

 

If you want to make a working boiler of an early loco, why not make it to scale dimensions externally, but capable of withstanding the heat (and the boiler tests!!!! – without too much hassle) and be practical and functional. Add the external straps at scale size but as dummies. It would look perfect and still work.  You could (soft) solder those on easily enough because the outer surface doesn’t get so hot.

 

Others may advise on boiler tests for that approach, but I believe that if the boiler is presented for test in the form it is to be used, the scale “embellishments” would be entirely neutral.  Better still talk to your club tester before starting out! (safety valves have to pass modern rules like the 10 minute accumulation test)

 

Huge satisfaction to be gained from such a project, and all credit.

[JasonBModerator @jasonb]

MGJ is right you can add detail to the external surface of a boiler without any problems, You can see in the picture below the dummy manhole and throatplate (brass sits) on the copper boiler for my traction engine.

 

 

Hopefully Kwil may chip in, he’s doing a large scale (7 1/4 I think) Locomotion but not sure if he’s done the boiler yet, the work he’s done so far is very good, having seen it in the flesh. 

Edited By JasonB on 14/04/2010 07:39:53

Edited By JasonB on 14/04/2010 07:40:41

[KWILParticipant @kwil]

Jason/Jon

 

Yes have the boiler completed, as locomotion does not have frames, you start with the boiler and hang everything on! I will bring it along in June, Jason. If I can sort out how to paste the photos here I will.

 

[CirclipParticipant @circlip]

If scale fidelity is so important Jonathan then cleading with Pb shim can cover the multitude of sins hidden by making a safe boiler.

 

  Come on mgj, nobody ever told you “You can’t scale nature” ? In steam terms you’ve obviously disproved the theory, so JasonBs tube plate should be remade to hold FAR more tubes of a more scale diameter.?

 

  Regards  Ian

[JasonBModerator @jasonb]

Kwil, judging by all those thickening plates there is certainly a lot going to be hung off that boiler.

 

Ian, I was planing to keep the smokebox door closed so no-one could count the tubes

 

And just for those that think a lapped and rivited boiler can’t be done here is a pic of a model being built at the moment.

 

Does help that it’s 6″ scale and being built in the US where it seems almost anything is allowed in some states regarding boiler construction. The rest of the build can be seen here for anyone interested, there is some very good fabrication work as its being done without castings, wish my welding was upto it.

 

Jason

Edited By JasonB on 14/04/2010 13:46:33

Edited By JasonB on 14/04/2010 13:47:10

[KWILParticipant @kwil]

The big plates carry the axle pedestals (horns) and the smaller plates are there to provide for screws which hold the various components, footplate supports, valve gear stantions, drawbar bracket etc. The pedestals are rivet located and are all hard soldered (silver)

[Ian S CParticipant @iansc]

There is an interesting artical in the first two issues of Model Engineer in 1898 about Dr. J. Bradbury Winter and his model D2 class engine named Como(this loco had been around a number of years at this stage).The throat-plate and fire box were made of steel. Instead of flanging them they were carved out of solid steelto a thickness of 1/96″,the machining only took 44hrs(hand shaper).The boiler had scale tubes ie 262 x 1/8″ dia. I can’t find the dimentions and thickness of metal for the boiler, but it was rivited.
Time to build 13000hrs. This was the first Model engineers workshop visited by Percival Marshall on starting “our journal”.
The first boiler is a 6″dia 9″ tall brass vertical boiler, made of 18 gauge/3/64″ brass, rivited and soldered. Pressure 15psi. A little about steam in 1898. Ian S C

[mgjParticipant @mgj]

Until said tiny tubes clog with ash and clinker.

 

I thought I was recommending the conventional route, precisely because of the problems of scaling nature, and because I could see no good purpose in trying to re-invent the wheel.

 

Perhaps I didn’t make myself clear.

Edited By mgj on 14/04/2010 18:26:43

[Jonathan HowesParticipant @jonathanhowes93303]

Mouthwatering pictures.

 

Regarding scaling nature. No, I would not attempt to run scale boiler tubes. grate areas need care as well if coal burning to give the appropriate draught intensity for combustion so this does not scale too well either. The engine I am contemplating has a double flue boiler and so is much closer in character to a model  in the first place. What it really comes down to is that I get a kick out of riveting little pressure vessels and it looks nice. In fact, since the materials at the time of the original locomotives construction were not that great the wall thicknesses are actually fairly thick even if directly scaled, just slightly thinner than shown on most model drawings. Phosphor bronze gives some hefty safety factors at the scale thickness so I wanted to know if there was a compelling reason not to go this route.

 

Jon. 

[JasonBModerator @jasonb]

I don’t know enough about the metals properties to say that bronze is or is not suitable, if you intend to run the loco anywhere but your own home then I would suggest you speak with a profesional boiler inspector before cutting metal as its him who will be testing the boiler.

 

Provided you can show him the calculations there should be no reason not to use a rivited and caulked boiler. Here for example is a vertical (copper) boiler constructed that way which has been tested and certified.

 

Steel boilers will scale down to a better thickness when using non ferrous as you don’t have to allow for wasting (corrosion) which happens at the same rate regardless of thickness. The copper boiler for my engien is 10swg but if it were going to be made in steel it would likely have to be 5swg.

 

Jason

[CirclipParticipant @circlip]

” There’s no such thing as can’t, but there may well be a different way.”

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